Plaque reduction neutralization tests were performed on several thousand human sera from Southeast Asia and the Pacific islands for evidence of antibodies to group A arboviruses. Antibodies against chikungunya virus were common among inhabitants of Southeast Asia, while Ross River virus antibodies were common in persons living in New Guinea and several of the Solomon Islands. Serologic studies were also done on acute and convalescent sera from volunteers infected with the Sicilian Phlebotomus fever virus. These studies indicate that persons infected with this virus develop high titers of specific neutralizing antibodies post-infection. Sera from residents of several Middle Eastern countries are currently being tested by plaque neutralization tests for antibodies to several Phlebotomus fever virus serotypes. The variation in susceptibility to chikungunya virus among different geographic strains of Aedes albopictus was studied. Marked differences in susceptibility to infection as well as in virus content were noted among various strains. Attempts to develop chikungunya resistant strains by genetic selection were unsuccessful. Transovarial transmission of La Crosse virus was demonstrated with experimentally infected A. albopictus. In contrast, vertical transmission did not occur with A. albopictus or Culex fatigans infected with Cache Valley, Batai, Arumowot, Itaporanga, VSV-Indiana, chikungunya or Ross River viruses.